Roberts Bay Condition Report for 2012

Summary:

The overall health of Roberts Bay has improved since 2010 with all water quality parameters below the threshold levels, the caution alert for chlorophyll a has been removed. However, levels of seagrass still remain below the target level.

Water quality: All three water quality indicators (chlorophyll a, nitrogen, and phosphorus) were rated as pass (below the threshold). The mean for chlorophyll a was calculated as an arithmetic mean and the means for nitrogen and phosphorus were calculated as geometric means (Numeric Nutrient Criteria Recommendations).The mean chlorophyll a level in Roberts Bay has improved since 2010 (0.0098 mg/l) and was scored as “Good” (scored as “Caution” in 2010), below the threshold (0.011 mg/l). The nitrogen level has increased, although the mean nitrogen level was below the threshold (0.540 mg/l), it exceeds the target level (0.450 mg/l) and was scored as “Good”. The phosphorus level has slightly increased, however the mean level (0.0923 mg/l) was still below the target level (0.190 mg/l) and was scored as “Excellent”.

Biotic Indicator: The total acreage of seagrass has remained relatively constant since 1988 but in 2010 the average level of seagrass (283 acres) was still below the target of 348 acres.

Dive Deeper!

Water Chemistry Ratings

Total nitrogen, total phosphorus, and chlorophyll a levels are monitored carefully by
water resource managers and used by regulatory authorities to determine whether a bay
meets the water quality standards mandated by the Clean Water Act. The trend graphs for
these indicators are shown below, along with their target and threshold values. A target
value is a desirable goal to be attained, while a threshold is an undesirable level which
is to be avoided. An individual indicator receives an "Excellent"
rating if its mean value is below the target, a "Good" rating if its mean value is above the target
but does not exceed the threshold, and a "Caution" rating if the mean value exceeds the threshold.
Learn More about these ratings and how they are calculated »

The charts below illustrate the general trend of water quality parameters. They show a six-month running average, which moderates high and low values in the data.

Chlorophyll a

The rating for Chlorophyll a is determined by comparing the annual arithmetic mean of its sampled values to fixed target and threshold values. The annual arithmetic mean is shown in the graph at left, as compared to its established target and threshold values.

Nitrogen, Total

The rating for Total Nitrogen is determined by comparing the annual geometric mean of its sampled values to target and threshold values. The target and threshold values are variable for Sarasota Bay, established annually, while they are fixed for the other bays. The geometric mean is found by multiplying together all the sample values for the calendar year and then taking the nth root of the result, where n is the number of samples. The annual geometric mean for Total Nitrogen is shown in the graph at left, as compared to its established target and threshold values.

Phosphorus, Total

The rating for Total Phosphorus is determined by comparing the annual geometric mean of its sampled values to fixed target and threshold values. The geometric mean is found by multiplying together all the sample values for the calendar year and then taking the nth root of the result, where n is the number of samples. The annual geometric mean for Total Phosphorus is shown in the graph at left, as compared to its established target and threshold values.

Other Measures of Bay Health

In addition to nutrient levels and chlorophyll concentration, dissolved oxygen levels, and water
clarity are also objective indicators of bay health. These have complex interactive cycles which
are affected by rainfall, temperature, and tidal action, as well as other factors. High nutrient
levels (nitrogen and phosphorus) can stimulate excessive growth of marine algae (indicated by
chlorophyll a level), resulting in reduced water clarity (and increased light attenuation) and
depleted oxygen levels. Both plants and animals in a bay need oxygen to survive, and the seagrasses
which provide food and cover for bay creatures need light for photosynthesis.

Bay Contour Maps (2012)

Contour mapping is one of the best ways to visualize spatial differences in coastal water quality.
The interactive map shown below presents monthly data for one selected water quality indicator atop
an aerial view of the bay. Choose a different water quality parameter from the list at the top to change the map.
Learn More about Water Quality Contour Mapping »

Visit the Water Quality Contour Mapping Tool to view and compare monthly water quality contour maps for ten different water quality indicators. In addition, you can generate your own custom maps.

Contour Legend:

Less than 1 mg/l

1.0 - 5.9 mg/l

6.0 - 10.9 mg/l

11.0 - 17.9 mg/l

Greater than 18 mg/l

Less than 300 ug/l

300 - 499 ug/l

500 - 699 ug/

700 - 899 ug/l

900 - 3,499 ug/l

Greater than 3,500 ug/l

Less than 100 ug/l

100 - 199 ug/l

200 - 299 ug/

300 - 399 ug/l

400 - 499 ug/l

Greater than 500 ug/l

Less than 4.0 ppt

4.0 - 10.0 ppt

10.0 - 18.0 ppt

18.0 - 35.0 ppt

Greater than 35.0 ppt

Seagrasses

Among the most important habitats in Florida's estuarine environments, seagrass beds are indispensable
for the role they play in cycling nutrients, supplying food for wildlife, stabilizing sediments, and
providing habitat for juvenile and adult finfish and shellfish. Use the interactive map below to
observe the size, density and location of seagrass beds from year to year. The graph shows how the total
amount of seagrass in the bay has changed over time. Seagrass calculations are aggregates of patchy and continuous seagrass measurements only. Recordings of attached algae are not included in these summaries.Learn More about Seagrasses »

Showing Seagrass Coverage for :

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Impervious Features

Rain that falls on land that is in a natural state is absorbed and filtered by soils and vegetation as it makes it way into underground aquifers. However, in developed areas, "impervious surfaces" impede this process and contribute to polluted urban runoff entering surface waters. These surfaces include human infrastructure like roads, sidewalks, driveways and parking lots that are covered by impenetrable materials such as asphalt, concrete, brick and stone, as well as buildings and other permanent structures. Soils that have been disturbed and compacted by urban development are often impervious as well.
Learn more about Impervious Features »

19% of the land area within the Sarasota Bay Watershed is covered by impervious surfaces

Land Use / Land Cover

Land use within a bay's watershed has a major effect on its water quality.
In general, less development means better water quality. Land Cover/Land Use
classifications categorize land in terms of its observed physical surface
characteristics (upland or wetland, e.g.), and also reflect the types of activity
that are taking place on it (agriculture, urban/built-up, utilities, etc.).
Florida uses as its standard a set of statewide classifications which were developed
by the Florida Department of Transportation.
Learn More about Land Use and Land Cover »

Roberts Bay is located within the Sarasota Bay Watershed. The chart below shows the land use / land cover characteristics for Sarasota Bay Watershed within the boundary of this Water Atlas. View details about the Sarasota Bay Watershed »

Acreage and Percentage within each Land Use / Land Cover Category for Sarasota Bay Watershed

Land Use Classification

1990

1995

1999

2005

2011

2014

Trend

Urban & Built-up

32,90853.3%

34,51755.9%

35,33457.3%

37,84461.3%

38,34362.1%

37,98761.6%

Agriculture

6,33810.3%

5,9459.6%

4,6467.5%

2,4974%

2,2153.6%

2,3093.7%

Rangeland

5470.9%

3370.5%

2970.5%

1990.3%

2250.4%

4300.7%

Upland Forests

3,5885.8%

2,8544.6%

2,6194.2%

2,1093.4%

1,8743%

1,9233.1%

Water

13,35021.6%

13,44521.8%

13,63322.1%

14,22723.1%

14,27823.1%

14,13122.9%

Wetlands

2,8704.7%

2,3793.9%

2,3523.8%

2,2273.6%

2,2293.6%

2,3723.8%

Barren Land

290%

1080.2%

7211.2%

90%

990.2%

1090.2%

Transportation and Utilities

1,8453%

2,1303.5%

2,1133.4%

2,6024.2%

2,4524%

2,4534%

Data Sources

The data sources listed below provided water quality data used to create the report on this page. Not all data sources provided data for every bay, and not every Bay Conditions Report used data from all listed data sources. While some data sources have no data for the scored year, they provided period-of-record (historical high, mean, low) data. Click on a data source name to review its metadata.

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